RFR: JDK-8310835: Address gaps in -Xlint:serial checks [v4]

[Joe Darcy]

> Improve serial lint messages for externalizable records (and potentially other situations).

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Refactor Externalizable check.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14639/files
  - new: https://git.openjdk.org/jdk/pull/14639/files/17b6dfd7..74a3055a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14639&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14639&range=02-03

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14639.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14639/head:pull/14639

PR: https://git.openjdk.org/jdk/pull/14639